A massive part of it was working out where the floor actually was. Brawn argued that the skid block section and the 'floor' should be considered individually and the 'holes' in the floor were simply gaps in the connecting planes.
What X is referring to is that these 'holes' that they had in there should be referred to as 'slots', gaps between the planes according to them. The viewing from below thing they came up with was nonsense because you could actually see the Brawn's suspension. As Rory Byrne, I think, said at the time:
"It's a play of words: (the three teams came up with) a section divided in three parts only to apply holes in the vertical intersection between the floor and the reference plane. But the regulations don't allow fully enclosed holes in the vertical intersection. There's even talk (in the regulations) of continuous, non-subdivided planes in the diffuser."
The other part to it was what was part of the diffuser and part of the rear crash structure.
Clearly all nonsense, but there we are. Mosley was declaring it legal before it had even been discussed. Given that they were the made illegal in the space of ten months for 2011 then you have to wonder why they were legal in the first place. The answer? It was a regulation minefield.